This invention relates to a method for the preparation of melamine by the conversion of urea and/or thermal decompositions products thereof in a fluidized bed of catalytically active material in the presence of a gas mixture containing ammonia and carbon dioxide.
One known process for the preparation of melamine at atmospheric pressure is described in Hydrocarbon Processing, September 1969 at pages 184-186. In that process, urea is reacted in a fluidized bed reactor in the presence of ammonia and carbon dioxide to form melamine. The melamine is recovered from the reaction gases by a "dry-catch" method using cooled gases to lower the temperature below the sublimation point of melamine. The off-gases leaving the desublimation step, freed of melamine and consisting mainly of ammonia and carbon dioxide, are partly recycled for use as fluidizing gas in the fluid bed reactor. However, prior to being recycled to the reactor, the off-gases are first subjected to a washing or scrubbing step with liquid urea, which scrubbing step is apparently there necessary to prevent an accumulation of impurities in the recirculated fluidizing gas.
During the scrubbing step, however, the urea is heated by the hot desublimator off-gases. In order to prevent undesirable side reactions, the temperature of the liquid urea must not be maintained at too high a level. Therefore a large amount of heat must be removed from the liquid urea at a relatively low temperature level in a liquid urea cooler, using cooling water.
Additionally, in such a urea scrubbing step, a portion of the liquid urea is entrained in the scrubbed off-gas mixture in the form of droplets, which cause clogging of downstream lines and equipment. These entrained urea droplets must therefore be removed from the off-gas mixture in a mist separator, or the like, which itself increases operating and maintenance requirements, and presents a risk of greater operating difficulties such as incrustration.
It is an objective of the present invention to provide an improved process for the preparation of melamine wherein the manner of recycling the desublimation off-gas mixture to the reactor is simplified. It is a further objective of this invention to provide an improved method for the preparation of melamine wherein the aforementioned difficulties encountered in the urea scrubbing step are eliminated.